correctly support Py_HASH_EXTERNAL for Unix platforms

Author: picnixz

Doc/c-api/hash.rst

@@ -44,6 +44,29 @@ See also the :c:member:`PyTypeObject.tp_hash` member and :ref:`numeric-hash`.
       Add :c:macro:`!Py_HASH_SIPHASH13`.
 
 
+.. c:macro:: Py_HASH_EXTERNAL
+
+   If :c:macro:`Py_HASH_ALGORITHM` is set to that value, the hash function
+   definition ``PyHash_Func`` must be provided by embedders at compile time.
+   For instance, to use SipHash-4-8 for conservative security purposes
+
+   .. code-block:: c
+
+      static Py_hash_t
+      siphash48(const void *src, Py_ssize_t src_sz) { ... }
+
+      PyHash_FuncDef PyHash_Func = {
+         .hash = siphash48,
+         .name = "siphash48",
+         .hash_bits = 64,
+         .seed_bits = 128,
+      };
+
+   .. availability:: Unix
+
+   .. versionadded:: 3.4
+
+
 .. c:macro:: Py_HASH_CUTOFF
 
    Buffers of length in range ``[1, Py_HASH_CUTOFF)`` are hashed using DJBX33A

Doc/using/configure.rst

@@ -1024,13 +1024,14 @@ Libraries options
 Security Options
 ----------------
 
-.. option:: --with-hash-algorithm=[fnv|siphash13|siphash24]
+.. option:: --with-hash-algorithm=[fnv|siphash13|siphash24|external]
 
    Select hash algorithm for use in ``Python/pyhash.c``:
 
    * ``siphash13`` (default);
    * ``siphash24``;
-   * ``fnv``.
+   * ``fnv``;
+   * ``external``.
 
    .. versionadded:: 3.4
 

configure

@@ -3966,12 +3966,15 @@ dnl quadrigraphs "@<:@" and "@:>@" produce "[" and "]" in the output
 AC_ARG_WITH(
   [hash_algorithm],
   [AS_HELP_STRING(
-    [--with-hash-algorithm=@<:@fnv|siphash13|siphash24@:>@],
+    [--with-hash-algorithm=@<:@fnv|siphash13|siphash24|external@:>@],
     [select hash algorithm for use in Python/pyhash.c (default is SipHash13)]
   )],
 [
 AC_MSG_RESULT([$withval])
 case "$withval" in
+    external)
+        AC_DEFINE([Py_HASH_ALGORITHM], [0])
+        ;;
     siphash13)
         AC_DEFINE([Py_HASH_ALGORITHM], [3])
         ;;